Semiconductor VLSI chips can contain in excess of a million transistors, together with hundreds of I/O (input/output) pads. To each of the pads, one or more chip-to-chip metallic interconnections ("interconnects") are attached, whereby a VLSI package is formed. The package typically contains as many as eight or more of these thus interconnected semiconductor VLSI chips.
The above-mentioned (metallic) interconnects are typically copper strips ("lines" or "wires"), each of which runs along one of two or more metallization levels (horizontal planes), each level being separated from the next by a suitable insulating layer, such as a layer of polyimide. Wires that are located on two successive metallization levels are connected to one another through holes (apertures; "vias") in the insulating layer filled with a suitable metal. Each via thus has a height that is equal to the thickness of the insulating layer, typically in the approximate range between 5 and 10 .mu.m, and the cross section of a via is typically 10 to 200 .mu.m in diameter. Each of the wires on the top level typically is connected to one or more I/O pads of one or more chips, typically by means of a glob ("bump") of solder.
The paper entitled, "High Density Interconnect for Advanced VLSI Packaging" by A. C. Adams et al. published in Diffusion Processes in High Technology Materials, Proceedings of the ASM Symposium, pp. 129-136 (October 1987), describes a VLSI package in which the wires are made of copper, because of its desirably high electrical conductivity, and the insulating layer is polyimide because of its excellent dielectric and mechanical properties. Because the polyimide does not adhere well to copper, however, if a copper wire comes in direct physical contact with the polyimide, then undesirable delamination of the polyimide from the copper wire would occur, whereby the structure would be mechanically unreliable. Moreover, because of potential chemical interaction of copper with polyimide, if copper in either the wires or the vias would be in direct physical contact with the polyimide, then the insulating properties of the polyimide would be deteriorated. On the other hand, because nickel has good adherence properties with respect to, and is desirably non-interactive with, polyimide; therefore, the aforementioned paper teaches that the copper wires are to be coated with nickel, and the vias are to be filled also with nickel (to form nickel "plugs").
More specifically, to obtain such a structure, the nickel could be deposited into the apertures by two electroless steps--i.e., successive immersions in aqueous solutions ("plating baths") containing nickel ions, one such immersion before the polyimide layer has been formed, and the other such immersion after the polyimide layer has been formed and has been supplied with the apertures. The first immersion would coat the copper wires with nickel; the second immersion would thus produce the nickel plugs. However, we have found that the required compositions of the plating baths for the two immersions must be different. More specifically, a plating bath that is suitable for electroless plating the copper wires with nickel is not suitable for filling the vias with nickel (plugs), especially in view of the required height of each nickel plug to fill each via. Thus, the resulting electroless deposited nickel formed during the first immersion is necessarily dissimilar in composition to the nickel deposited during the second immersion, whereby the (second) nickel in the vias deposits poorly (if at all) on first nickel layer that coats the (top and side) surfaces of the copper; consequently, the subsequently formed nickel plugs undesirably do not reliably fill the vias. At the same time undesirably poor adhesion of copper to the polyimide tends to result, whereby moisture can undesirably migrate from the environment to the various levels and undesirably cause corrosion of the metallization.
Another approach is forming a thin nickel layer on copper by means of a quick ("flash") electroless process in a first plating bath, and using the resulting thin nickel layer as a foundation for a second, thick nickel layer deposited on this thin nickel layer by means of a second electroless process in a second plating bath having a composition that is different from that of the first bath, followed by forming the polyimide layer with its apertures filled with nickel plugs formed by means of an electroless process in a third plating bath having the same composition as that of the second bath. We have found that such a ("flash") electroless nickel layer also tends not to be a reliable foundation for the thick nickel layer and hence for the subsequent formation of the nickel plugs, again because of the required different composition of the two plating baths-one for the thin ("flash") nickel layer, and the other for the (overlying) thick nickel layer. That is, the thick nickel layer, as deposited on the thin ("flash") nickel layer, tends to be non-uniform in thickness, whereby at least some of the vias undesirably are not filled with metal and hence at least some of the desired electrical connections between successive metallization levels are undesirably nonexistent.
Moreover, we have found that the use of electroplating (battery-assisted plating) as a process for coating the copper wires with nickel tends to produce an electroplated layer (of nickel on copper) that has an undesirably nonuniform thickness. Also, electroplating cannot be used at all to form the nickel plugs because at the time these plugs are to be formed it is simply not feasible to electrically access all the copper wires.
Therefore, it would be desirable to have a method of plating nickel on copper (wires)--the nickel having properties that enable reliable electroless formation of nickel, for example, in an aperture in an overlying insulating layer.